Method and apparatus for replacement of underground pipe

ABSTRACT

An impactor for use with a directional boring machine for replacement of underground pipes includes a bursting head having a rearwardly opening recess and a front end opening that communicates with the recess. An inlet pipe is connected at its front end to a distal end of the drill string and extends into the recess of the bursting head through the front end opening. A striker is mounted for sliding movement along the inlet pipe, and a distributing mechanism responsive to pressure fluid supplied through the drill string and inlet pipe causes the striker to reciprocate in the rearwardly opening recess of the head to deliver forward impacts against the bursting head. The striker and distributing mechanism are mounted in a tubular housing, and a pulling connection is provided by which the impactor can be pulled by the drill string while the striker is delivering impacts against the head in the same direction as a pulling force exerted by the drill string. The bursting head is preferably slidably mounted on the inlet pipe and moves forward relative to the inlet pipe and drill string in response to an impact of the striker against the bursting head. The impactor can be readily used with existing directional boring machines.

This application claims priority of U.S. Provisional Patent ApplicationSer. No.: 60/348,824, filed Jan. 14, 2002.

TECHNICAL FIELD

This invention relates to systems for underground pipe bursting andreplacement.

BACKGROUND OF THE INVENTION

A variety of systems are now known for the installation of undergroundpipes, particularly for the replacement of an existing deterioratedpipe. Pneumatic impact tools that bore horizontal holes or burstexisting pipelines are in widespread use. See, for example, Wentworth etal. U.S. Pat. No. 5,025,868. In one widely practiced method, a pneumaticimpact boring tool is sent through the existing pipeline such that thehead of the tool, which may be provided with blades that apply intenselocal pressure to the existing pipe, fractures or splits the existingpipe. See, for example, Streatfield et. al. U.S. Pat. Nos. 4,720,211,4,738,565 and 4,505,302. A replacement pipe, typically made of plasticsuch as HDPE, is drawn along behind the boring tool. This process hasproven effective commercially because it bursts the old pipe andreplaces it with a new pipe at the same time. However, exhaust from theimpact tool is vented into the interior of the replacement pipe, whichis unacceptable for certain types of pipe installations, such as gas andwater lines.

Directional drilling machines are less effective for pipe bursting,especially for hard to burst pipes like cast iron, because the steadypushing force of the drill string lacks the impact power of a pneumaticimpact boring tool. Thus, in some instances, a directional borer orwinch is used to pull a pneumatic impact tool through an existingpipeline in order to burst the existing pipe and pull in the replacementpipe. These alternatives are effective but require considerableequipment and manual labor.

Wentworth U.S. Pat. No. 5,782,311 describes a pipe bursting apparatususing a directional boring machine capable of simultaneously rotatingand pulling a drill string and an impact tool connectable to the drillstring. The impact tool includes a tubular housing ending in a frontbursting head, an input shaft, bearings supporting the input shaft forrotation within the housing, a striker disposed for reciprocation withinthe housing to deliver impacts to a front anvil, and a drive mechanismfor simultaneously pulling the impact tool forward in response to apulling force on the input shaft and for reciprocating the striker todrive the tool forward by the action of cyclic impacts on the frontanvil. The apparatus can carry out a pipe bursting operation in a mannerthat eliminates the need to use a separate pneumatic impact tool toburst the existing pipe. The present invention provides an impactorwhich operates using a pressure fluid such as compressed air, and whichcan be readily used with existing directional boring machines.

A number of cable operated static bursting systems have been used forpipe bursting. Carter et al. PCT Publication WO 98/30350 describes apipe bursting system that makes use of a relatively small, light weighthydraulic cable puller to pull a pipe bursting mole. Fisk et al. U.S.Pat. Nos. 4,983,071 and 5,078,546 describe a cable-drawn tool backed byan impact tool and provided with a blade for slitting and expanding asteel gas main. Wentworth et al. U.S. Pat. No. 6,269,889 illustratesanother bursting method wherein a pneumatic tool drawn by a conventionalwinch is used for pipe bursting. In general systems, that rely only onstatic pulling force either cannot handle large pipe sizes or require avery large, powerful pulling system that is expensive and laborintensive to use. System combining pulling force with an impact toolhave been able to handle larger pipe sizes than comparable static forceonly systems, but difficulties have been encountered with starting andstopping the tool during a run. When the tool is far down the bore, itcan be difficult to re-start the tool. The present invention addressesthis disadvantage among others.

SUMMARY OF THE INVENTION

An impactor of the invention for use with a directional boring machinehaving a drill string includes a bursting head having a rearwardlyopening recess and a front end opening that communicates with therecess, an inlet pipe connected at its front end to a distal end of thedrill string and extending into the recess of the bursting head throughthe front end opening, a striker mounted for sliding movement along theinlet pipe, a distributing mechanism responsive to pressure fluidsupplied through the drill string and inlet pipe to cause the striker toreciprocate in the rearwardly opening recess of the head to deliverforward impacts against the bursting head, a tubular housing in whichthe striker and distributing mechanism are mounted, a pulling connectionby which the impactor can be pulled by the drill string while thestriker is delivering impacts against the head in the same direction asa pulling force exerted by the drill string, and a valve which regulatessupply of pressure fluid to the distributing mechanism. The burstinghead is preferably slidably mounted on the inlet pipe and moves forwardrelative to the inlet pipe and drill string in response to an impact ofthe striker against the bursting head. The tubular housing, needed toprovide sealed pressure chambers, may comprise a separate tool body, anextension of the bursting head, or a front end portion of the plasticreplacement pipe, as explained hereafter.

In a preferred form of the invention, a valve is provided in the inletpipe which can shut off flow of pressure fluid, such as in response torelaxation of pulling force on the drill string that changes therelative position of the inlet tube and the bursting head. Thedistributing mechanism expels exhaust air rearwardly out of the tubularhousing, and typically suitable means is provided for pulling thereplacement pipe along behind the bursting head. For example, thereplacement pipe may be secured in the rearwardly opening recess of thehead, such that a front end portion of the replacement pipe forms thetubular housing.

The distributing mechanism of the impactor may be of any suitable type,but one preferred mechanism includes a rearwardly opening recess in thestriker, a radial outlet hole in the striker communicating with therecess, a rear seal bearing by which the striker slidingly, sealinglyengages an inner surface of the tubular housing, a radial outlet hole inthe inlet pipe permitting pressure fluid to enter the rearwardly openingrecess in the striker from the inlet pipe, and a sleeve mounted on theinlet pipe rearwardly of the radial outlet hole therein in sliding,sealing contact with an inner surface of the striker inside the recess,forming a rear pressure chamber in the recess ahead of the sleeve.Pressure fluid in the rear pressure chamber causes a forward stroke ofthe striker resulting in an impact against the bursting head, and causesthe radial outlet hole in the striker to move past a front edge of thesleeve. This in turn causes pressure fluid from the rear pressurechamber to pass outwardly to a front pressure chamber outside of thestriker between the striker and the tubular housing, thereby causing thestriker to move rearwardly until the radial outlet hole in the strikermoves past a rear edge of the sleeve. The front pressure chamber thenexhausts, and a new forward stroke of the striker begins. The sleeve ofthe distributing mechanism may be mounted on the inlet tube, so that norear assembly for securing the distributing mechanism to the tubularhousing is needed. “Radial” in the context of the invention means in adirection leading outwardly from a central axis, but is not limited to adirection that is perpendicular to a central axis. A radial outlet holecould, for example, also be slanted in a lengthwise direction.

An alternative pneumatic impact mechanism according to the invention canbe used to advantage in a pipe bursting impactor and can be adapted toimpact tools of other types, such as underground piercing tools and jackhammers. Such a mechanism includes as general components a tubular bodyhaving a rearwardly opening recess and a front end opening thatcommunicates with the recess, an inlet pipe connectable at its front endto a source of pressure fluid and extending into the recess of the bodythrough the front end opening, a striker mounted for sliding movementalong the inlet pipe and an inside surface of the body for deliveringforward impacts, the striker having a rearwardly opening recess and afront central opening of smaller diameter than the recess and whichcommunicates with the recess, through which front opening the air inletpipe extends, and a distributing mechanism responsive to pressure fluidsupplied through the inlet pipe to cause the striker to reciprocate in acycle. The distributing mechanism includes a radial hole in the inletpipe permitting pressure fluid from inside the inlet pipe to enter therearwardly opening recess in the striker, a sleeve mounted at a rear ofthe inlet pipe in slidable, sealing relationship with a wall of therearwardly opening recess in the striker, forming a pressure chamber fordriving the striker forwardly, a first passage for permitting pressurefluid from the rear pressure chamber to enter a front pressure chamberahead of the striker to move the striker rearwardly after an impact,which passage is isolated from the rear pressure chamber during a partof the striker cycle in which the striker is propelled forwardly, and asecond passage for exhausting the front pressure chamber after thestriker has moved rearwardly a sufficient distance. Such a mechanismavoids the need for providing a radial hole through a thin tubular wallof the striker, which has been a frequent cause of breakage in “stepbushing” style underground piercing tools going back to Sudnishnikov etal. U.S. Pat. Nos. 3,410,354 and 3,756,328.

According to another aspect of the invention, a pulling adapter isprovided which can be attached to the rear end of the bursting head. Theadapter has a sealing wall therein which prevents exhaust from thedistributing mechanism from entering the replacement pipe, and exhaustholes permitting exhaust from the distributing mechanism to pass outsideof the pulling adapter and replacement pipe. If the pulling adapter ismade of plastic, it can be coaxially welded to a leading end of thereplacement pipe.

The invention further provides an apparatus for use with a directionalboring machine having a drill string for widening an existing hole. Sucha system includes a head having a front nose portion and a rear,hole-widening portion of greater external diameter than the noseportion, and a ball joint in front of the nose portion configured forconnecting the head to a drill string and permitting swiveling of thehead relative to the drill string. The head may, for example, be animpactor that is pulled without rotation, or a back reamer that isrotated while being pulled to widen the existing hole. The ball jointmay be provided with a passage therethrough for passing pressure fluidfrom the drill string to operate a pneumatic impactor. An adapter may beprovided having a front end portion configured for connection to aleading end of the drill string and a mechanical coupling that connectsa rear end portion of the adapter to a front end portion of the balljoint.

The invention in another aspect provides methods for replacement of anexisting pipeline using a directional boring machine having a drillstring and an impactor mounted at a terminal end of the drill string. Inone such method, the impactor includes a bursting head, a striker whichdelivers impacts to the bursting head, and a pressure fluid-actuatedimpact mechanism. The method comprises:

(a) inserting the drill string through the pipeline;

(b) connecting a distal end of the drill string to the impactor;

(c) pulling the drill string and impactor into the pipeline whiledrawing a replacement pipe behind the impactor with a front end openingof the replacement pipe in a sealed condition;

(d) operating the impactor as needed to burst the pipeline by supplyinga pressure fluid to operate the impactor, the pressure fluid including alubricant; and

(e) injecting exhaust pressure fluid including the lubricant from theimpactor outside the impactor and replacement pipe. A baffle or spoutmay be used to direct the pressure fluid rearwardly towards thereplacement pipe. This both avoids contaminating the inside of the pipewith the exhaust and aids in installing the new pipe.

In another method for replacement of an existing pipeline of theinvention, the pressure fluid-actuated impact mechanism has a valveregulating supply of pressure fluid to the impactor. The method includesthe steps of:

(a) inserting the drill string through the pipeline;

(b) connecting a distal end of the drill string to the impactor;

(c) pulling the drill string and impactor into the pipeline whiledrawing a replacement pipe behind the impactor;

(d) actuating the impactor by pulling on the drill string, which causesthe valve to supply pressure fluid to the impact mechanism; and

(e) stopping the impactor by reducing pulling force on the drill string.This eliminates the need to manually stop and start the impactor duringbursting operation.

The invention further provides a method and apparatus for replacement ofan underground pipe wherein a cable or other flexible pulling implement(such as a chain) can be used to pull the impactor. An impactoraccording this aspect of the invention includes a bursting head having arearwardly opening recess, a striker mounted for sliding movement withinthe head, a distributing mechanism responsive to pressure fluid suppliedthrough an inlet to cause the striker to reciprocate in the rearwardlyopening recess of the head to deliver forward impacts against the head,and a pulling connection by which the impactor can be pulled while thestriker is delivering impacts against the head in the same direction asa pulling force. A length of a flexible pulling implement such as acable is engagable to the connection at the front end of the impactorfor delivering the pulling force. A cable pulling system pulls the cablein a plurality of repeated cyclic pulling strokes. Such a systemincludes one or more holders such as collets that engage the cable andone or more cylinders each having a movable piston operative to move theholders along the pulling stroke. In a preferred embodiment, an inletpipe extends into the recess of the head for supplying compressed air tothe distributing mechanism, and a valve in the inlet pipe shuts off flowof pressure fluid to the distributing mechanism when closed. The pullingconnection may be disposed at a front end of the inlet pipe, whereby thevalve is opened by pulling on the pulling connection with a pullingdevice and closed by relaxation of pulling force exerted by the pullingdevice.

A method for replacement of an underground pipe using such an apparatusincludes the steps of:

(a) inserting a cable through an existing pipeline from a first end to asecond end thereof;

(b) connecting a free end of the cable extending from the second end ofthe pipeline to an impactor, which impactor includes a bursting headhaving a rearwardly opening recess, a striker mounted for slidingmovement within the head, a distributing mechanism responsive topressure fluid supplied through an inlet to cause the striker toreciprocate in the rearwardly opening recess of the head to deliverforward impacts against the head, and a pulling connection to which thecable is connected;

(c) pulling the impactor with the cable using a cable pulling systemdisposed at the first end of the pipeline, which cable pulling systempulls the cable in a plurality of repeated cyclic pulling strokes andincludes a holder that engages the cable and a cylinder having a movablepiston operative to move the holder along the pulling stroke;

(d) operating the impactor to deliver impacts against the head in thesame direction as the pulling force during a cyclic pulling stroke; and

(e) reducing operation of the impactor between cyclic pulling strokes sothat the impactor does not substantially move between strokes.“Reducing” in this context means lessening the impact force delivered bythe striker to the bursting head, generally by reducing the supply ofcompressed air or pressure fluid to the impactor partially orcompletely, as explained further hereafter. These and other aspects ofthe invention are discussed in the detailed description that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, like numerals represent like elementsexcept where section lines are indicated:

FIG. 1 is a schematic diagram of an installation according to a methodof the invention;

FIG. 2 is an enlarged view of the impactor shown in FIG. 1;

FIG. 3 is a lengthwise section taken along the line 3—3, showing thestriker in contact with the anvil surface of the bursting head;

FIG. 4 is the same view as FIG. 3, with the striker retracted to itexhaust position;

FIG. 5 is the same view as FIG. 3, with the bursting head in position toshut off the air valve;

FIG. 6 is a partial, rear end view of an alternative embodiment of theimpactor of the invention;

FIG. 7 is a lengthwise section along the line 7—7 in FIG. 6;

FIG. 8 is a side view of a second embodiment of an impactor according tothe invention in an inoperative position;

FIG. 9 is a lengthwise sectional view of the impactor of FIG. 8;

FIG. 10 is a side view of the second embodiment of an impactor accordingto FIGS. 8 and 9, in an operative position;

FIG. 11 is a lengthwise sectional view of the impactor of FIG. 10;

FIG. 12 is a cross sectional view taken along the line 12—12 in FIG. 10;

FIG. 13 is a cross sectional view taken along the line 13—13 in FIG. 10;

FIG. 14 is a cross sectional view taken along the line 14—14 in FIG. 10;

FIG. 15 is a side view, partly in lengthwise section, of the impactor ofFIGS. 8 through 14 in a bent position;

FIG. 16 is a rear view of the impactor of FIG. 15;

FIG. 17 is an enlarged view of the ball joint and valve mechanism shownin FIG. 9;

FIG. 18 is a schematic diagram of a method of removing an impactoraccording to the invention from a manhole;

FIG. 19 is a side view of a back reamer provided with a ball jointaccording to the invention;

FIG. 20 is a side view of an alternative impactor according to theinvention which can be pulled by a cable or chain;

FIG. 21 is a lengthwise sectional view taken along the line 21—21 inFIG. 20;

FIG. 22 is a cross sectional view taken along the line 22—22 in FIG. 20;

FIG. 23 is a cross sectional view taken along the line 23—23 in FIG. 20;

FIG. 24 is a cross sectional view taken along the line 24—24 in FIG. 20;

FIG. 25 is a lengthwise sectional view of the impactor of FIG. 20, at a90 degree angle to FIG. 21;

FIG. 26 is the same view as FIG. 21, but showing the valve in a closedposition;

FIG. 27 is a schematic diagram of a further method of the inventionusing the impactor of FIGS. 20-26;

FIG. 28 is a front view of a further embodiment of an impactor accordingto the invention;

FIG. 29 is a lengthwise section taken along the line 29—29 in FIG. 28;

FIG. 30 is a lengthwise section taken along the line 30—30 in FIG. 28;

FIG. 31 is the same view as FIG. 29, taken with the striker in aretracted position;

FIG. 32 is the same view as FIG. 30, taken with the striker in aretracted position;

FIG. 33 is the same view as FIG. 29, taken with the valve in an offposition;

FIG. 34 is the same view as FIG. 30, taken with the valve in an offposition; and

FIG. 35 is an enlarged view of a midportion of FIG. 34.

While the making and using of various embodiments of the presentinvention are discussed in detail below, it should be appreciated thatthe present invention provides many applicable inventive concepts whichcan be embodied in a wide variety of specific contexts. The specificembodiments discussed herein are merely illustrative of specific ways tomake and use the invention and are not to limit the scope of theinvention.

DETAILED DESCRIPTION

Referring to FIG. 1, a pipe bursting and replacement system 10 accordingto the invention includes an impactor 11 pulled by a directionaldrilling machine 12 by means of a drill string 13. Impactor 11 ispositioned at a starting location, such as a manhole or entrance pit 14,and pulled through an existing pipeline 16, bursting it into fragments17 which remain in the ground. The replacement pipe 18 is drawn alongbehind impactor 11 and occupies the same space as the existing pipeline.

Impactor 11 as shown in FIGS. 2-5 includes an impact mechanism that aidsin boring through the ground or bursting an existing pipe. Impactor 11includes a central air inlet tube 21 that may be connected eitherdirectly or by means of an adapter 22 to the terminal end of drillstring 13. A conical shell or bursting head 23 is mounted on the outsideof tube 21 in close sliding contact therewith. A shoulder 24 leading toan enlarged outer diameter rear portion of tube 21 engages a slantedstep 26 on the inner surface of head 23 so that pulling the drill string13 pulls head 23 forward into contact with the existing pipeline orborehole. The replacement pipe 18 fits into a rear end opening 27 inhead 23 and is secured therein by fasteners such as screws insertedthrough radial holes 28 through a rear cylindrical portion of head 23.Pipe 18 is typically made of plastic, and as such screws used for thispurpose tap directly into pipe 18. For purposes of the invention, pipe18 may comprise the front end portion of the actual pipe 18 or aseparate plastic tool body (which may be slightly thicker than pipe 18as illustrated) to which an end of pipe 18 is welded, as described incommonly assigned Wentworth U.S. Pat. No. 6,269,889, issued Aug. 7,2001, the entire contents of which are incorporated by reference herein.

A striker 31 is mounted at its front end on the outer surface of tube 21rearwardly of head 23. During operation, a pressure fluid such ascompressed air fed through tube 21 can be used to reciprocate striker31, causing it to deliver rapid impacts to a rearwardly facing inneranvil surface 32 of head 23. For this purpose, compressed air is fedfrom machine 12 through drill string 13 and into tube 21. Tube 21 has afront central flow passage 33 that leads to an internal valve 34. Valve34 controls operation of the impact mechanism. Front flow passage 33ends as shown in one or more radial passages 36 which form part of valve34. In the position shown in FIG. 3, passages 36 communicate with anannular groove 37 on the inner surface of head 23. Compressed air canflow from passages 36 through groove 37 to a second set of radialpassages 38 rearwardly of passages 36. Passages 38 pass compressed airin turn to a rear flow passage 39 in tube 21 through to a series of rearradial passages 41 located towards the rear end of tube 21, which end issealed as by a cap 42.

By this means, the compressed air is directed into a rear pressurechamber 43 formed by a rearwardly opening recess 45 in striker 31 and asleeve 44 secured on tube 21 rearwardly of holes 41. Seals 46 mounted inannular grooves on the outside of sleeve 44 are in sliding, sealedengagement with the inner surface of striker 31. As a result, compressedair in chamber 43 can only escape through radial ports 47 into anannular front (variable) pressure chamber 48. An annular seal 51 on arear outer surface of striker 31 prevents compressed air from escapingfrom front chamber 48. As a consequence, the greater frontal surfacearea of striker 31 causes the net force exerted by the compressed air tomove the striker 31 rearwardly until ports 47 pass over the rear edge ofsleeve 44 as shown in FIG. 4. At this point, the front pressure chamber48 exhausts into the interior of the replacement pipe 18, the pressurein constant pressure chamber 43 becomes greater than the pressure infront chamber 48, and striker 31 is propelled forwardly until it impactsagainst anvil surface 32. The cycle then repeats as long as the valve 34remains open. In this embodiment, the impact mechanism is sized to fitclosely inside of pipe 18 and use pipe 18 as the impact tool body,eliminating the need for an expensive steel tool body. Unlike strikerscommonly used in pneumatic ground piercing tools, striker 31 slidesalong the outside of tube 21 but preferably does not engage the insideof pipe 18 or a corresponding metal tool body anywhere other than atrear seal 51.

Bursting head 23, unlike the body of a typical pneumatic piercing tool,is preferably free to move forward relative to the rest of impactor 11in response to the impact it receives from striker 31 over a shortdistance D. An enlarged diameter front portion of the adapter 22 definesa stop for the front end 56 of head 23, but in practice distance D isselected so that head 23 does not move forward more than a very limiteddistance, preferably slightly less than distance D, typically about 0.1to 0.5 inch during normal operation. The resistance of the ground and/orexisting pipeline ahead of head 23, together with the inertia of thereplacement pipe 18, limits such forward movement. Decoupling head 23from the drill string 13 is preferred in the invention because a fixedexpander would permit impacts from the striker to be directlytransmitted to the drill string, damaging both drill string 13 anddirectional boring machine 12. To the extent this movement closes airvalve 34 fully or partially, the constant pulling force exerted by thedrill string after the impact will reopen valve 34.

In the alternative, if the expander is not moveable relative to theinlet tube, then it is preferred to interpose a soft link or cablebetween the terminal end of the drill string and the front of impactor11, so that the impact force is not transmitted back through the drillstring during an impact, but the impactor 11 can still be pulled by thedrill string. For this purpose, it may be necessary to provide both acable connection (for pulling) and a separate air hose for conductingcompressed air from the drill string to the head, with resultinginconvenience to the operator.

For some boring operations, such as installation of a gas or water line,it may be desirable to avoid contaminating the inside of the replacementpipe with the impactor exhaust, which carries with it oil and othercontaminants. For this purpose, as shown in FIGS. 6 and 7, a separatetool body 71 which may be all plastic, or may comprise a front metalportion 76 coupled to a rear plastic portion 77 by screws, threads, orthe like. Plastic rear portion 76 has a sealing end wall 78 and one ormore radial holes 79 just ahead of wall 72. In the alternative, theleading end of the replacement pipe can be towed along behind theimpactor in a conventional manner, with a removable cap secured over itsopen end to prevent contamination. By such means, during the exhaustpart of the cycle, compressed air leaving the front pressure chamber canescape to the outside of pipe 18. An overhanging flange 74 or one-wayvalve mechanism can be used to prevent debris from entering throughholes 79 during periods when space 81 ahead of end wall 78 is notpressurized. If tool body 71 or rear portion 77 is plastic of the sametype as pipe 18, the two may be welded together end to end and latersawed apart when the run is finished as described in Wentworth U.S. Pat.No. 6,269,889.

According to a preferred version of this embodiment, the pressure fluidused to power the impact mechanism includes a foaming agent of a typeknown in the art which causes compressed air to turn into a lubricantfoam upon depressurization. The exhaust will then become a foam on theoutside of the pipe 18 that lubricates it and eases its passage into theground, improving boring efficiency due to less friction between thereplacement pipe and the ground, and reducing damage to the new pipe asit is dragged through the ground with fragments of the old pipe aroundit.

When the unit is running, valve 34 is open as discussed above and shownin FIGS. 3 and 4. However, when the drill string stops exerting apulling force on impactor 11, head 23 moves to its frontwardmostposition as shown in FIG. 5. Annular groove 37 becomes isolated fromrear radial passages 38, cutting off the supply of compressed air to theimpact mechanism and causing the striker to stop. The impact mechanismthen restarts when pulling force exerted by the drill string once againcauses the valve to open as shown in FIGS. 3 and 4. In this manner, themovement of head 23 over a limited range serves a dual purpose in actingboth as a shutoff valve and protecting the drill string from impacts.

In operation, such as shown in FIG. 1, drill string 13 is first sentthrough the existing pipeline to the starting location. This ispreferably accomplished by operating the directional drilling machine 12with a conventional drill bit to bore from a first surface location 15on the ground surface to the exit location, such as an exit pit ormanhole 19. The drill string is then extended through the existingpipeline 16 to the entry location, such as an entry pit or manhole 14.Here the bit is reattached, and the boring machine drills upwardly to asecond surface location 20. If necessary, a deflection plate positionedat the bottom of manhole 14 can be used to direct the drill string 13upwardly at the desired angle, or chains pulled by a winch can be usedto pull the drill string upwardly in manhole 14. When the bit and drillstring reach the second surface location 20, the bit is removed andreplaced by impactor 11, and replacement pipe 18 is attached to the backof impactor 11 by screws or welding as discussed above, or by othersuitable means such as a pipe pulling adapter.

The drill string is then pulled back down to the entry pit or manhole 14to begin the run. Machine 12 is then operated to pull impactor 11 backto the exit location 19 as shown while supplying compressed air or otherfluid through the inside of the drill string to operate the impactmechanism. Each time a new section of drill rod must be added, valve 34causes the impact mechanism to stop automatically in response to thereduced tension on the drill string as described above, without need forthe operator to control the operation of the impactor manually. This isa key advantage of the present invention in that virtually all pipebursting systems presently in commercial use operate intermittently, notcontinuously. The impactor must stop while the operator adds or removesanother rod to the drill string, or between strokes in the case ofhydraulic pulling systems. When impactor 11 emerges into the exit pit ormanhole 19, it can be disconnected from replacement pipe 18, and theentire mechanism pulled out of the pipe and removed through the mouth ofmanhole 19.

The impact mechanism of the invention can deliver powerful impacts witha much simpler impact mechanism than systems that rely on rotation ofthe drill string, spring force or the like to propel the striker. Theuse of foam injected outside of the hole of the replacement pipe willlubricate the replacement pipe, making it easier to pull. The foregoingstructure decoupling the head so that it can move relative to the drillstring greatly reduces wear on the drill string and boring machine, butwithout need for additional connectors such as a soft link.

FIGS. 8 to 17 illustrate an alternative impactor 211 according to theinvention. Impactor 211 includes a central air inlet tube 221 that maybe connected either directly or by means of an adapter 222 to theterminal end of drill string 13. However, in this embodiment, adapter222 is connected to tube 221 by a ball joint 201 described hereafter. Astepped conical shell or bursting head 223 is mounted on the outside oftube 221 in close sliding contact therewith. A shoulder 224 of tube 221engages a slanted step 226 on the inner surface of head 223 so thatpulling the drill string 13 pulls head 223 forward into contact with theexisting pipeline or borehole. The replacement pipe 18 fits into a rearend opening 227 in head 223 and is secured therein by fasteners such asscrews inserted through radial holes 228 through a rear cylindricalportion of head 223. In this embodiment, head 223 has sufficient lengthto house the entire impact mechanism; the replacement pipe is not usedas the tool body.

A striker 231 is mounted at its front end on the outer surface of tube221 rearwardly of head 223. During operation, as in the precedingembodiment, a pressure fluid such as compressed air fed through tube 221reciprocates striker 231, causing it to deliver rapid impacts to arearwardly facing inner anvil surface 232 of head 223. For this purpose,compressed air is fed from machine 12 through drill string 13, adapter222, and ball joint 201 into tube 221. Tube 221 has a front central flowpassage 233 that leads to an internal valve 234 that controls operationof the impact mechanism. Front flow passage 233 ends in one or morerearwardly angled, radial passages 236 which form part of valve 234.Passages 236 communicate with an annular groove 237 on the inner surfaceof head 223. Compressed air can flow from passages 236 through groove237 to a second set of radial passages 238 rearwardly of passages 236when in the open position shown in FIG. 11. Passages 238 pass compressedair to a rear flow passage 239 in tube 221 through to a series of rearradial passages 241 located towards the rear end of tube 221, which endis sealed as by a cap 242. By this means, the compressed air is directedinto a rear pressure chamber 243 formed by a rearwardly opening recess245 in striker 231 and a sleeve 244 secured on tube 221 rearwardly ofholes 241. Seals 246 mounted in annular grooves on the outside of sleeve244 are in sliding, sealed engagement with the inner surface of striker31. The rear end of tube 221 is sealed by a cover plate 247 that issecured by bolts 249 to the back of sleeve 244.

At this point, the impact mechanism differs substantially from the priorembodiment. No radial ports through the striker similar to ports 47 areprovided. Instead, air inlet tube 211 has a front, enlarged diameterportion 301 that sealingly engages the inside of head 223. Passages236,238 open at the front and rear ends of front portion 301,respectively. When impactor 11 is in its off position as shown in FIG.9, an annular passage 302 extending frontwardly between the outside oftube 221 and the inside of striker 231 is sealed off from a frontvariable volume pressure chamber 248, effectively providing a secondvalve for stopping the impactor when required. When tube 221 is pulledforward by the drill string to the position shown in FIG. 11, passage302 is now capable of supplying compressed air from the rear chamber 243to the front of the striker, i.e. to front chamber 248. A rear, enlargeddiameter portion 303 of tube 221, preferably provided with a sealingring 304, also sealingly engages the inside of head 223 to seal offpassage 302 at certain times in the cycle. A rearwardly-openingcounterbore or a number of parallel grooves 306 in the wall of passage302 which act as bypass passages permitting compressed air to flow pastenlarged portion 303 when the striker has delivered its impact as shownin FIG. 9, causing the striker to move rearwardly. Front chamber 248becomes isolated from rear chamber 243 when portion 303 reaches thefront end of counterbore 306.

The front pressure chamber exhausts when a rear annular seal 251 on theoutside of the striker clears one or more lengthwise exhaust passages307 machined on the inside of head 223 as shown in FIG. 15. The pressurein rear pressure chamber 243 then becomes greater than the pressure infront chamber 248, and the impact cycle repeats. For purposes ofpermitting the space ahead of the striker 231 to communicate withexhaust passages 307, striker 231 has one or more lengthwise grooves 309through the head portion of the striker that engages the inside of head223 rearwardly of the tapered nose 310 thereof and a reduced diameterrear portion forming an annular air passage 311 which extends back toexhaust passage 307. Seal 251 and rear portion 303 of tube 221 are shownas threaded-on rings, but unless adjustability is needed, are preferablyintegral with their respective base parts.

The striker continues to reciprocate in this manner until tension isrelaxed on the drill string, which causes the gap between the head 223and ball joint 201 to close, closing valve 234 and cutting off thesupply of compressed air (or other fluid) to rear chamber 243. Exhaustpassages 307 may if desired by directed radially or at an anglerearwardly to direct the exhaust outside of the bursting head, insteadof into the replacement pipe, for the reasons discussed above. Passages307A (dotted), replacing passages 307, may be located for this purpose.

Ball joint 201 toward the front of impactor 211 provides flexibility foroccasional bending of the device relative to the drill string and aidsin removal of impactor 211 from a man-made confined space such as amanhole, where enlarging the space to remove the head would entaildamage an existing structure. Ball joint 201 includes a cylindrical,frontwardly opening socket 320 of the same diameter as a front end ofhead 223, a ball assembly 321 disposed at the bottom of socket 320, anda pin 325 for coupling the ball joint 201 to a widened rear end portion218 of adapter 222. A front end portion 219 of adapter 222 has internalor external threads as appropriate for direction connection to a leadingend of the drill string. As shown in FIG. 17, ball assembly 321comprises front and rear sections 322, 323 coupled by bolts 324.Sections 322, 323 have aligned, threaded holes therethrough which permitball joint 201 to be threadedly secured to external threads on a frontend portion of tube 221. Tightening bolts 324 with a slight separationbetween sections 322, 323 applies a clamp load to each of the threadedconnections and makes ball joint 201 more secure, yet more easilyremoved upon loosening of bolts 324.

After ball assembly 321 and socket 320 have been mounted on tube 221 asshown, a rear end of adapter 222 is inserted into socket 320 until atransverse hole 326 therein is in alignment with a pair of like-sizedtransverse holes 327 in socket 320. Pin 325 is then inserted into holes326, 327 to couple adapter 222 and ball joint 201 together. Pin 325 maybe press-fit therein or secured by a further retaining pin, such as arolled pin, oriented perpendicularly to the length of pin 325 andextending through a hole 330 in pin 325 and corresponding holes insocket 320 and adapter 222 in alignment with hole 330. Pin 325 has ahole 329 therethrough which aligns with a bore 331 of adapter 222 andpassage 233 for passing compressed air or other pressure fluid to theimpact mechanism. A front end surface 332 of adapter 222 is rounded likean inner retaining surface 333 of socket 320 to match the curvature ofball assembly 321. A slight clearance remains between adapter 222 andball assembly 321 through which compressed air can enter the ball joint,but the steel-to-steel contact of ball assembly 321 with surface 333 ofsocket 320 provides a sufficient seal when the impactor is running.

Ball joint 201 provides a number of advantages. When the bursting headenters the existing pipe, the drill string will be bent at an angle. Theball joint makes it easier to align the bursting head with the existingpipe, and makes it easier for the head to follow slight changes ofdirection in the existing pipe without becoming jammed. Referring toFIG. 18, once the device enters an exit manhole 319, the impactor 211must be removed and withdrawn through the existing manhole opening 341.For this purpose, impactor 211 must be removed from both the drillstring 13 (at the front) and the replacement pipe 18 (at the back.) Anybox-pin style joint requires moving the affected parts axially apartfrom one another. The manhole 319 may be too narrow to permit this whenimpactor 211 is straight (coaxial) relative to the drill string.According to a method of disassembly according to the invention, thedrill string is first pulled forward far enough so that the screws setin openings 228 are accessible, and impactor 211 is disconnected fromreplacement pipe 18 by removing the screw, bolts, or other fasteners. Atthis point, ball joint 201 may actually be a few inches outside of themanhole on the opposite side. Head 223 is then tipped up and over theend of pipe 18, and the drill string 13 is then moved back a shortdistance (e.g., six to ten inches or so), exposing joint 201 in themanhole as shown. The joint is then disassembled by removal of pin 325,drill string 13 is withdrawn with adapter 222 attached, and impactor 211is lifted out of manhole 319 through opening 341 by a winch or the like.This greatly facilitates use of the bursting system in urban areas wheresewer lines connected to manholes require bursting and replacement.

Ball joint 201 has a substantial diameter and is generally usable onlywhere the inner diameter of the existing pipeline is relative large,i.e., at least larger than the outer diameter of the ball joint.However, a second smaller expander can be positioned ahead of joint 201to provide it with sufficient clearance. Similarly, a ball joint of theinvention could also be used to connect a back reamer of similar shapeto head 223 to a drill string, but without an internal impactor, for usein backreaming to widen an existing hole. For example, as shown in FIG.19, adapter 222 and ball joint 201 are mounted in front of aconventional back reamer 340 by means of a threaded projection 341 atthe front of reamer 340 that is secured in a threaded opening 342 at therear end of ball joint 201. In a double back reamer assembly where afirst, smaller diameter back reamer is connected to the drill string anda second, larger diameter back reamer is connected to the rear of thefirst one, the ball joint of the invention can be provided as the meansfor connecting the second reamer to the back of the first, so that thedouble reamer system would be able bend along its length when necessary.

FIGS. 20-26 illustrate a further embodiment of a pull-to-run impactor411 according to the invention which has elements in common with priorembodiments. Central air inlet tube 421, bursting head 423, tube 421,shoulder 424, step 426, rear end opening 427, radial holes 428, striker431, anvil surface 432, annular groove 437, rear pressure chamber 443,sleeve 444, rearwardly opening recess 445, seal 446, cover plate 447,front pressure chamber 448 and bolts 449 are essentially the same asdescribed above using the corresponding reference numerals 211-249respectively, except as otherwise noted. Impactor 411 is designed to bepulled by a cable or chain rather than a string of rods, and as suchreceives compressed air through a hose that runs inside the replacementpipe and conducts compressed air to a rear end opening 452 of a second,inner inlet tube 451 disposed inside of and coaxially with tube 421. Forthis purpose, cover plate 431 has a central hole 450 therein throughwhich inner and outer tubes 421, 451 extend. Compressed air or otherpressure fluid flows forwardly through a flow passage 453 which is theinterior of tube 451 to radial holes 456 near the front end of tube 421which holes communicate with aligned holes 457 in outer tube 421.

When in the position shown in FIG. 21-25 with the pull to run valveopen, compressed air enters groove 437 and flows back through passages458 in outer tube 421 that communicate with an annular flow passage 459rearwardly of holes 456, 457 and between tubes 421 and 451. Thecompressed air passageway, which has essentially doubled back on itself,then leads out through rear radial ports 461 in outer tube 421 and intorear pressure chamber 443. At this point compressed air can flowforwardly past seal 462 through grooves 463 and into annular passage 464in the same manner as described above for passage 302 and groove 306,initiating rearward movement of striker 431. Exhausting of the frontpressure chamber 448 occurs when the front ends of rear seals 466 onstriker 431 move past the front ends of exhaust grooves 467.

The resulting rearrangement permits use of a rear compressed air supplyin combination with a front valve that remains open when a pulling forceis applied but closes to the position shown in FIG. 26 when the pullingforce is released. For purposes of attachment, a front end portion ofouter tube 421 has a suitable connecting portion, such as a clevis 471,for connecting a cable. A press-fit pin 472 may be inserted throughaligned transverse holes in inner and outer tubes 421, 451 to hold thetubes together. Protruding ends 473 of pin 472 can act as a stopdefining the frontmost position of the head 423. As shown in FIG. 26,the valve as it closes chokes off the supply of compressed airgradually, and in some cases the valve may not close completely,allowing the tool to continue to run at low power.

According to a further aspect of the invention, a cable-operated staticbursting system is provided substantially as described in Carter et al.PCT Publication WO 98/30350 and U.S. Pat. No. 6,305,880, Issued Oct. 23,2001, the entire contents of which applications are incorporated byreference herein. An impactor 411 of the invention is substituted forthe mole described, with the air supply hose running back through theplastic replacement pipe.

Referring to FIG. 27, an existing burstable pipe 514 is in the ground518 below the surface 520. An entry first hole (not shown) is dug toexpose a first end of pipe 514, and a second hole or exit pit 532 is dugexposing the second or exit end 536 of pipe 514. Impactor 411 isinserted nose first into the first end of pipe 514. A length ofreplacement pipe 560 is attached to the rear end of impactor 411 usingscrews as described above. Impactor 411 is also connected to a flexiblecompressed air hose 471 running through the inside of replacement pipe560 which supplies compressed air from a compressor 472 located on thesurface near the entry pit. A mole pulling cable 570 is passed throughexisting pipeline 514 and secured to clevis 471 by a pin or bolt.

In many cases, cable 570 is made of braided steel strands and is stiffenough to be fed through existing pipeline 560 manually prior to thestart of the run. In other instances, especially in sewer or water lineswherein the inside of the pipe may be partially clogged with deposits,the cable 570 can be fed by attaching it to a suitable guide, such as afiberglass rod or plumbing snake, if necessary with an auger-like actionif needed to clear a path for the cable, and feeding the guide throughthe pipeline 514 from the exit pit to the entry pit. The guide is thenwithdrawn and the cable 570 may be attached to impactor 411.

A cable pulling system 580 is disposed in the exit pit 532. System 580includes a pulling frame 584 having legs 588 and a cable pulley 592attached thereto. A reaction plate 596 is placed against a side wall 600of the hole 532 to provide a secure reaction surface. Cable 570 passesthrough a slot in plate 596 and around pulley 592 to a cable pullingdevice 620. Cable pulling device 620 is removably mounted in a slottedmounting annulus 624 that has been secured to frame 584. Device 620 ispreferably a dual-cylinder hydraulic cable puller equipped with colletsoperatively connected to the cylinders for pulling the cable a shortdistance per stroke. A second set of collets is provided inside ofannulus 624 to hold the cable 570 in tension between strokes. Such asystem is sold commercially by TRIC Trenchless Ltd. The cylinders ofdevice 620 are connected to a hydraulic pump 628 by a pair of hydrauliclines 632. In the alternative, for jobs requiring less pulling force,the cylinders of device 620 can be pneumatic cylinders connected byhoses to an air compressor that takes the place of pump 628.

The described hydraulic system pulls the impactor 411 in a series ofintermittent strokes. Between strokes, various different means such ascollets or clamping jaws can be used to hold the cable in tension. Assuch, impactor 411 may continue to run for a short time after the strokeis done, but will stop or diminish in power once the bursting head ispushed forward relative to the tube connected to the cable, which is nolonger subject to a constant pulling force. This is useful in that itconserves compressed air and prevents the tool from overrunning thecable between strokes, possible resulting in the cable becoming tangled.The pull-to-run valve of the invention also makes restarting the toolwhen it is far underground much easier in comparison to a valve locatedon the surface near the air compressor.

FIGS. 28 to 34 illustrate a further embodiment of the invention whereinthe air flow is modified so that the primary air supply passage, ratherthan being closed off when the pulling force on the drill string isrelaxed, exhausts to the atmosphere instead. This embodiment also has adifferent valve structure that provides a large volume of air flow in arelatively short impactor. The described impactor 711 receivescompressed air from the drill string, but could be modified to be pulledby a chain or cable and receive compressed air from a hose connected toits rear end.

FIGS. 28-32 show the impactor 711 in its normal operating mode, with apulling force exerted on the drill string. Impactor 711 includes acentral air inlet tube 721 that may be connected directly the terminalend of a drill string. A conical shell or bursting head 723, which actsas the housing for the impact mechanism, is mounted on the outside oftube 721 in close sliding contact therewith as in preceding embodiments.A shoulder 724 of to an enlarged outer diameter front portion 720 oftube 721 engages a slanted step 726 on the inner surface of head 723 sothat pulling the drill string pulls head 723 forward into contact withthe existing pipeline or borehole. However, unlike in earlierembodiments, enlarged diameter front portion 720 does not incorporatevalving passages. The replacement pipe fits into a rear end opening 727in head 723 and is secured therein by fasteners such as screws insertedthrough radial holes 728 through a rear cylindrical portion of head 723.A striker 731 is mounted at its front end on the outer surface of tube721 rearwardly of head 723. During operation, a pressure fluid such ascompressed air fed through tube 721 is used to reciprocate striker 731,causing it to deliver rapid impacts to head 723. For this purpose,compressed air is fed from machine 12 through drill string 13 and intotube 721. Tube 721 has an inner central flow passage 733 that leads to aseries of three equiangular radial valve holes 734 and is sealed at itsrear end by a central portion of a cap 742.

Compressed air is directed into a rear pressure chamber 743 formed by arearwardly opening recess in striker 731 and a sleeve 744 secured ontube 721. Sleeve 744 in this embodiment comprises a hollow cylinder.Holes 734 open inside of sleeve 744 and compressed air enters chamber743 through a circular formation of equiangularly spaced air passages745 in the front of sleeve 744. Cap 742 is secured by bolts to a plug749 that seals the rear end of sleeve 744. A seal 746 mounted in anannular groove on the outside of sleeve 744 is in sliding, sealedengagement with the inner surface of the rear opening in striker 731.

An annular passage 802 extending frontwardly between the outside of tube721 and the inside of striker 731 conducts compressed air to a frontvariable volume pressure chamber 748. A rear, enlarged diameter portion803 of tube 721, preferably provided with a sealing ring 804, sealinglyengages the inside of head 723 to seal off passage 802 at certain timesin the impact cycle. A rearwardly-opening counterbore or a number ofparallel grooves 806 in the wall of passage 802 which act as bypasspassages permitting compressed air to flow past enlarged portion 803when the striker has delivered its impact as shown in FIGS. 29 and 30,causing the striker to move rearwardly. Front chamber 748 becomesisolated from rear chamber 743 when portion 803 reaches the front end ofgrooves 806.

The front pressure chamber exhausts when a rear annular seal 751 on theoutside of the striker clears one or more lengthwise exhaust passages807 machined on the inside of head 723 as shown in FIGS. 31 and 32. Thepressure in rear pressure chamber 743 then becomes greater than thepressure in front chamber 748, and the impact cycle repeats.

The striker continues to reciprocate in this manner until tension isrelaxed on the drill string, which causes tube 721 to move rearwardlyrelative to head 723. Tube 721 has a number of flow passages 761 thatopen on enlarged diameter portion 720 and remain covered by the innersurface of head 723 (or a seal bearing set therein) during normaloperation. Following rearward movement of tube 721, the radial openings762 of these passages 761 become uncovered as shown in FIGS. 33 and 34.Compressed air from front chamber 748 then vents to the atmosphere, andair from rear pressure chamber 743 flows along passages 802 and is alsovented to the atmosphere through passages 761, which are aligned withcorresponding holes 766 in cap 742.

While striker 731 in its forwardmost position could seal off passages802 by sliding entirely over enlarged diameter front portion 720, inpractice it does not do so. Striker 731, acted on by compressed air onboth sides and still in motion from the previous cycle, moves to aposition where the forces acting on it are equal on both sides. For thispurpose it is important that part of enlarged diameter front portion 720extend rearwardly from opening 762. The size of the opening betweenenlarged diameter portion 720 and striker 731 at the end of passage 802is regulated by the striker's position and varies the amount of pressureon each side of the striker. In this manner the striker stops in asuitable position to resume cycling once enlarged diameter portion 720is pulled forward to cover openings 762 again.

Enlarged diameter front portion 720 of tube 721 again acts as a valvemember for stopping the striker, but does so in a manner that does notcut off the flow of compressed air through the tool. This permits theoverall length of the tool to be substantially reduced because, amongother reasons, enlarged diameter front portion 720 can be shorter thanin prior embodiments wherein angled inlet and outlet passages areprovided in order to cut off air flow. The shortened tool is more usefulwhen the impactor must be removed from a manhole having a narrow topopening as discussed above, and it may be possible to remove theshortened tool without need for a front ball joint.

According to a further aspect of the invention, it has been found thatthe metal on metal contact between striker 731 and sleeve 744 is afrequent cause of binding and failure unless steps are taken to ensure alooser but sealing fit. As illustrated in detail in FIG. 35, seal 746comprises a Teflon ring that is slightly larger than a groove 771 intowhich it fits, with a slight clearance beneath (0.03 inch in theembodiment shown.) A similar slight clearance is provided between theoutside of sleeve 744 and the inside of striker 731 to prevent binding.Seal 746 fits into groove 771 loosely enough to accommodate smallchanges of position of striker 731 relative to sleeve 744 in the radialdirection, but is large enough to prevent leakage through the clearancetherebetween. The use of an oversize seal ring and a groove withclearance beneath the seal ring, permitting some degree of movement byseal 746, prevents binding and permits use of a relatively long sleeve744. The front seal ring 804 is preferably a Teflon ring mounted in thesame manner. An elastomeric shock absorbing ring 772 is also providedbetween a shoulder 773 of tube 721 and the front of sleeve 744 to dampenshocks to tube 721.

While certain embodiments of the invention have been illustrated for thepurposes of this disclosure, numerous changes in the method andapparatus of the invention presented herein may be made by those skilledin the art, such changes being embodied within the scope and spirit ofthe present invention as defined in the appended claims. A number ofaspects of the invention have wide applicability and can be used inother types of pneumatic impact tools, such self-propelled impact groundpiercing tools. “Pressure fluid” should be understood to includecompressed air and other forms of pressure fluids that could be used fora like purpose, such as other gases or liquids.

1. An impactor, comprising: a head having a rearwardly opening recessand a front end opening that communicates with the recess; an inlet pipeconnectable at its front end to a pulling device and extending into therecess of the head through the front end opening, and having a pullingconnection by which the impactor can be pulled while the striker isdelivering impacts against the head in the same direction as a pullingforce exerted by the pulling device; a striker mounted for slidingmovement along the inlet pipe; a distributing mechanism which receivespressure fluid supplied through the inlet pipe to cause the striker toreciprocate in the rearwardly opening recess of the head to deliverforward impacts against the head; and a valve which regulates supply ofpressure fluid to the distributing mechanism, including a front portionof the inlet pipe which assumes a first position in response to pullingforce exerted on the inlet pipe by the pulling device that causes thedistributing mechanism to reciprocate the striker, and a second positionin which relaxing of the pulling force on the inlet pipe causes thedistributing mechanism to cease reciprocating the striker, and a passagethat permits pressure fluid in the inlet pipe to exhaust to theatmosphere when the valve is in the second position.
 2. The impactor ofclaim 1, further comprising means for moving the inlet pipe towards thesecond position when the pulling force on the inlet pipe is relaxed. 3.The impactor of claim 1, wherein the distributing mechanism furthercomprises a front pressure chamber in front of the striker and a rearpressure chamber behind the striker, and the passage that permitspressure fluid in the inlet tube to exhaust to the atmosphere when thevalve is in the second position comprises a lengthwise passage thatopens at a rear end portion of the inlet tube and extends forwardly tocommunicate with a radial port that is covered by the head in the firstposition and uncovered in the second position, such that pressure fluidfrom the inlet pipe flows from the front pressure chamber through theradial port into the passage.
 4. The impactor of claim 3, whereinpressure fluid from the inlet pipe flows through a central flow passageand at least one radial hole in the inlet pipe into the rear pressurechamber, and from the rear pressure chamber through an annular openingbetween the striker and head into the front pressure chamber.
 5. Theimpactor of claim 4, wherein the passage that permits pressure fluid inthe inlet pipe to exhaust to the atmosphere when the valve is in thesecond position is formed in a wall of the inlet pipe and runs inparallel with the central flow passage.
 6. The impactor of claim 4,further comprising a sleeve mounted on the inlet tube, and the strikerhas a rearwardly opening recess in which the sleeve is in sliding,sealing contact, whereby the sleeve and the striker cooperate to formthe rear pressure chamber, and the inlet tube further comprises at leastone radial hole that opens inside the sleeve and wherein the sleeveincludes at least one passage communicating with the rear pressurechamber such that pressure fluid flows through the radial hole into therear pressure chamber.
 7. The impactor of claim 1, further comprising asleeve slidably disposed in a rearwardly opening recess in the strikerwith an external surface of the sleeve having an annular groove thereinwith a seal bearing disposed in the groove, the seal bearing beingoversized relative to the groove such that an annular space existsbeneath the seal bearing at the bottom of the groove, and an outerportion of the seal bearing extends from the groove to form a sealbetween the external surface of the sleeve and the recess of thestriker.
 8. The impactor of claim 1, wherein the head is slidablymounted on the inlet pipe and moves forward relative to the inlet pipeand drill string in response to an impact of the striker against thehead when the inlet tube is in the first position.
 9. The impactor ofclaim 8, wherein a front portion of the inlet pipe comprises an enlargeddiameter portion which engages a shoulder on the head when the inletvalve is in the first position.
 10. The impactor of claim 1, wherein theimpactor further comprises means for pulling a replacement pipe alongbehind the head.
 11. The impactor of claim 1, wherein the head has afrontwardly tapering external surface suitable for pipe bursting.
 12. Apneumatic impact mechanism, comprising: an elongated head including atubular housing; a striker disposed for reciprocation within an internalchamber of the housing to impart impacts to a rear impact surface of thehead for driving the impact mechanism forwardly through the ground; andan air distributing mechanism disposed for effecting reciprocation ofthe striker, including a sleeve slidably disposed in a rearwardlyopening recess in the striker with the external surface of the sleeveseparated from the rearwardly opening recess of the striker by a slightclearance, wherein an external surface of the sleeve has an annulargroove therein with a seal bearing disposed in the groove, which sealbearing is oversized relative to the groove such that an annular spaceexists beneath the seal bearing at the bottom of the groove, and anouter portion of the seal bearing protrudes from the groove intosliding, sealing contact with the recess of the striker.